All text between a ! character and the end of line is treated as a
comment. Comments are most useful when entered into a script. Since
there is no difference between reading from the terminal or reading
from a script, comments can also be entered interactively.

There are two types of comments. If the first character in the line
is ! then the entire line is ignored and the next line is read. If !
is in the input line, but is not the first character, the ! and all
following characters are removed. Thus if the software requires that
you enter a blank line (i.e., default) and you wish to include a
comment, then you must include one or more spaces before the !.

Examples:

xsel > show ! This is a comment
xsel > ! this line is completely ignored
xsel > ! this causes the program to read a blank line